Microwave photonic links have been studied extensively due to their growing applications at high frequencies in commercial and defense communications such as CATV, antenna remoting, avionics, synthetic aperture radar, phased array antenna, etc. Radio frequency (RF) photonics is attractive for both transmission and signal processing. RF signal processing applications such as channelizing receivers provide the opportunity for optics to perform the challenging task of transforming wideband spectral processing at high frequencies in the RF domain into narrowband processing in the optical domain with reduced complexity.
Analog photonic links employ either intensity modulation or phase modulation for electrical-to-optical (E/O) conversion and are limited in linearity primarily due to the sinusoidal transfer function of the intensity modulator or due to the nonlinear optical-to-electrical (O/E) conversion at the receiver for phase modulated signals. The two-tone spurious free dynamic range (SFDR) is limited mainly by third-order intermodulation distortion (IMD), which falls within the signal band.
The utility of analog links depends upon various parameters including link gain, noise figure, bandwidth, and link linearity or dynamic range. High dynamic range is key to achieving high fidelity analog links and places highly challenging requirements on the components and the design of the system. The linearity of the link can be characterized by the SFDR and is primarily dependent on the modulation and detection scheme. Both intensity modulation using direct detection and phase modulation using either direct detection or coherent detection have been studied. All these links exhibit a nonlinear transfer function and are thus limited in their linearity. Besides modulation and detection, the characteristic of the nonlinearity also depends on whether the link employs filtering.
Both optical and electronic methods to extend the dynamic range have been proposed and demonstrated. Electronic methods involve electronic predistortion while optical methods include cascaded modulators for predistortion, optical spectrum shaping, optical phase locked loops (PLL), and coherent post-processing. Significant reduction in the intermodulation distortion has been achieved using these methods. Predistortion techniques require knowledge of the nonlinearity and may employ adaptive circuits to track changes in the input signal. Predistortion techniques are limited in their efficacy when multiple distortion mechanisms are present as they require as many separate circuits to correct for the nonlinearities as the number of distortions.
Post-processing methods, which are extensively used in applications such as software defined radio (SDR), can correct simultaneously for multiple distortions. Advantageously, digital signal processing (DSP) capability is already present in many applications to perform standard processing functions. However, the post-processing methods, such as an electro-optical PLL, rely on having access to the entire modulated signal in order to compensate for link nonlinearity and reconstruct the original transmitted signal. However, RF signal processing applications channelize a smaller spectrum of the modulated signal thus precluding these techniques.